A resistivity cross-section of Usu volcano, Hokkaido, Japan, by audiomagnetotelluric soundings

Abstract

We collected audio-magnetotelluric (AMT) data across Usu volcano, Hokkaido, Japan, which erupted in 1977 and is still active. We had a profile of 17 sites perpendicular to the regional tectonic strike, which crossed the 1977 cryptodome, Usu-Shinzan. Tensor-decomposed data were interpreted by a two-dimensional inversion. Outside the crater rim, the resistivity structure is simple. The resistive somma lava is underlain by a conductive substratum, implying altered Tertiary or Quaternary rocks. In the crater, there are two resistive bodies bisected by a vertical conductor, which corresponds to Usu-Shinzan fault, located at the foot of the uplift. The vertical conductor was not detected in the AMT sounding in 1985. One of the possible causes of the development of the vertical conductor is a cold water supply from the surface into the vapor dominant fracture zone. One of the resistive bodies is located beneath Usu-Shinzan and implies an intrusive magma body which caused the 1977 uplift.

Mathematical simulation of magma-hydrothermal activity associated with the 1977 eruption of Usu volcano

“…More recently, MT measurements revealed the underground structure of Usu volcano (Nishida et al, 1996;Ogawa et al, 1998). Figure 2 shows a two-dimensional resistivity section crossing Usu volcano from northeast to southwest (Matsushima et al, 2002).…”

“…Kagiyama, 1984), and the volume of heat source was estimated (Matsushima, 1992). The 1977 intrusion was imaged by a recently conducted magnetotelluric study (Ogawa et al, 1998;Matsushima et al, 2002). Effective permeabilities and the hydrological structure of Usu volcano were estimated using wells drilled for hot spring exploitation at the foot of the volcano (Oshima and Matsushima, 1999).…”

Inter-eruptive volcanism at Usu volcano: Micro-earthquakes and dome subsidence

“…Katsui et al (1985) performed a topographic survey around the summit area to estimate the location of the magma that swelled Usu-Shinzan and discussed the magma intrusion process beneath Usu-Shinzan. Later, the magma body was clearly imaged at a depth of around sea level by magnetotelluric surveys (Matsushima et al, 2001, Ogawa et al, 1998. While a shallow part of the volcano to depths of 2-4 km is a conspicuous conductor (1-500 ・・m), a resistive block of 500-1000 m is situated beneath Usu-Shinzan at depths of 300-500 m below sea level (Matsushima et al, 2001).…”

“…The hypocenters of the precursory earthquakes of the 2000 eruption were mainly located at depths of 2-3.5 km below sea level (Onizawa et al, 2007). The shallow intrusion of magma during the 1977-1982 eruption was estimated under Usu-Shinzan by magnetotelluric surveys (Ogawa et al, 1998, Matsushima et al, 2001.…”

Magnetotelluric and temperature monitoring after the 2011 sub-Plinian eruptions of Shinmoe-dake volcano

“…In particular, we have already used the MT method to evaluate the large-scale subsurface resistivity structure present within the seismogenic zone beneath the Japanese Archipelago Island Arc system (Ogawa et al, 2001;Uyeshima et al, 2005;Yoshimura et al, 2009), and to evaluate the smaller-scale (shallow) resistivity structure beneath a number of historically active Japanese volcanoes (Ogawa et al, 1998;Hase et al, 2005;Aizawa et al, 2009b;Kanda et al, 2010).…”